1. Technical Field
The invention relates to a driving apparatus for a photo-addressing type display element, a driving method and the photo-addressing type display element.
2. Related Art
Various rewritable marking arts high in convenience are researched. As one direction, in recent years, attention has been focused on a display element using cholesteric liquid crystal because it has the features of having a memory property capable of retaining display with no power supply, providing bright display because of using no polarizing plate, being capable of producing color display without using any color filter, and the like.
A planar texture indicated by cholesteric liquid crystal (chiral nematic liquid crystal) causes a selective reflection phenomenon to occur in which light incident in parallel with a screw axis is separated into right optical rotation and left optical rotation, the circular polarization component matching the twist direction of the spiral is Bragg-reflected, and the remaining light is allowed to pass through. Letting the helical pitch be p, average refractive index be in the plane orthogonal to the screw axis be n, and complex refractive index be Δn, center wavelength λ of reflected light and reflection wavelength width Δλ are represented as λ=n·p and Δλ=Δn·p respectively and reflected light based on the cholesteric liquid crystal in the planar texture produces vivid color dependent on the helical pitch.
The cholesteric liquid crystal having positive dielectric constant anisotropy shows three states of a planar texture in which the screw axis becomes perpendicular to the cell surface and the above-mentioned selective reflection phenomenon is caused to occur for incident light as shown in FIG. 20 (A), a focal conic texture in which the screw axis becomes almost parallel to the cell surface and incident light is allowed to pass through while it is a little dispersed forward as shown in FIG. 20 (B), and a homeotropic texture in which the spiral structure comes loose and liquid crystal director is oriented in the electric field direction for allowing incident light to pass through almost completely as shown in FIG. 20 (C).
Of these three states, the planar texture and the focal conic texture can exist bistably in no electric field. Therefore, the phase state of the cholesteric liquid crystal is not uniquely determined with respect to the electric field strength applied to a liquid crystal layer and if planar texture is the initial state, the state changes in the order of the planar texture, the focal conic texture, and the homeotropic texture with an increase in the electric field strength; if focal conic texture is the initial state, the state changes in the order of the focal conic texture and the homeotropic texture with an increase in the electric field strength.
On the other hand, if the electric field strength applied to the liquid crystal layer is set to zero suddenly, the planar texture and the focal conic texture maintain the state intact and the homeotropic texture changes to the planar texture.
Therefore, the cholesteric liquid crystal layer just after a pulse signal is applied shows a switching behavior as shown in FIG. 21 and when the voltage of the applied pulse signal is Vfh or more, the selective reflection state in which the homeotropic texture changes to the planar texture is entered; when the voltage is between Vpf and Vfh, the transmission state based on the focal conic texture is entered; and when the voltage is Vpf or less, the state in which the state before the pulse signal is applied is continued, namely, the selective reflection state based on the planar texture or the transmission state based on the focal conic texture is entered.
In FIG. 21, the vertical axis is normalized light reflectivity; the light reflectivity is normalized with the maximum light reflectivity set to 100 and the minimum light reflectivity set to 0. Since a transition region exists among the states of the planar texture, the focal conic texture, and the homeotropic texture, the case where the normalized light reflectivity is 50 or more is defined as selective reflection state and the case where the normalized light reflectivity is less than 50 is defined as transmission state and the threshold voltage of texture change of the planar texture and the focal conic texture is Vpf and the threshold voltage of texture change of the focal conic texture and the homeotropic texture is Vfh.
FIG. 22 is a schematic drawing to schematically show a state where an image is written to a display element with an exposure device. As shown in FIG. 22, the display element is provided by depositing a display layer of a liquid crystal layer and an organic photosensitive layer of a photoconductive layer between a pair of transparent electrodes (sandwiching a shading layer (not shown) as required) and sandwiching them between a pair of substrates. The surface on the side of the organic photosensitive layer is exposed to light by the exposure device like an image in a state where a predetermined bias voltage is applied to both the transparent electrodes, whereby any desired record image can be written.
This display element can form a full-color image if units each sandwiching the display layer and the photoconductive layer between the electrode layers are deposited as three colors of R, G, and B.